1. Field of the Invention
The present invention relates to power tools, and in particular, to an automatic gear shifting power tool in which a speed reduction ratio is changed in accordance with a torque.
2. Description of the Related Art
Japanese Patent Application Publication No. 06-008151 discloses a power tool of an automatic gear shifting type. The power tool comprises a prime mover, a tool shaft driven by the prime mover and a gear reducer disposed between the prime mover and the tool shaft. The gear reducer is equipped with a planetary gear mechanism composed of a sun gear, a planet gear, an internal gear and a carrier.
In the gear reducer, the internal gear of the planetary gear mechanism is movably installed between a first position and a second position along an axial direction. When the internal gear is located in the first position, the internal gear and the sun gear are coupled together so as to be integrally rotated. On the other hand, when the internal gear is located in the second position, the internal gear is non-rotatably fixed. When the torque applied to the tool shaft is less than a predetermined value, the internal gear is retained in the first position, and when the torque applied to the tool shaft reaches or exceeds the predetermined value, the internal gear is moved to the second position. The gear reducer further includes a spring which biases the internal gear toward the first position when the internal gear is located on the first position side, and biases the internal gear toward the second position when the internal gear is located on the second position side.
According to the above-described configuration, as long as the torque applied to the tool shaft is less than the predetermined value, the planetary gear mechanism is maintained in a non-functional state in which a high-speed (low-torque) operation is performed. On the other hand, after the torque applied to the tool shaft has reached or exceeded the predetermined value, the planetary gear mechanism is shifted to a functional state in which a low-speed (high-torque) operation is performed. In other words, speed reduction ratio of the gear reducer is switched at a time when the torque applied to the tool shaft has reached or exceeded the predetermined value.